Fig. 4. (A) Chemical structure of the photosensitiser RuP. (B and C) Electron transfer mechanisms from the photoexcited RuP dye to the catalyst CoPⁿ in the homogenous and heterogeneous suspension systems with TiO₂ and ZrO₂ particles. The ‘through particle’ electron transfer pathway proceeds through oxidative quenching of RuP and is only accessible in RuP|TiO₂|CoPⁿ (see text). (D) Schematic energy diagram with the redox potentials of RuP⁺/RuP* and RuP/RuP^– generated upon photoexcitation, conduction band potentials of the semiconductor particles (TiO₂-CB and ZrO₂-CB), the thermodynamic redox potential for proton reduction, E^0’(H⁺/H₂), and the catalytic proton reduction onset potentials, E_cat, of the CoPⁿ catalysts determined from CVs in TEOA/Na₂SO₄ (0.1 M each, pH 7) and acetate electrolyte solution (0.1 M, pH 4.5).